[Bug-moe] Build failure with latest clang compiler

[FX]

Hello,

Latest clang (from macOS 10.13 / Xcode 9) is stricter about ambiguous 
overloaded functions. And thus it fails to build moe 1.9. The error here is in 
window_vector.cc (see full error below).

Cheers,
FX Coudert



----------------------------------------

window_vector.cc:1033:21: error: call to member function 'insert' is ambiguous
      { year_string.insert( 0U, 1, '0' + ( year % 10 ) ); year /= 10; }
        ~~~~~~~~~~~~^~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1035:19:
 note: candidate function
    basic_string& insert(size_type __pos, size_type __n, value_type __c);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1046:9:
 note: candidate function [with _InputIterator = int]
        insert(const_iterator __pos, _InputIterator __first, _InputIterator 
__last);
        ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1038:19:
 note: candidate function
    iterator      insert(const_iterator __pos, size_type __n, value_type __c);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1032:19:
 note: candidate function not viable: no known conversion from 'int' to 'const 
std::__1::basic_string<char>' for 2nd argument
    basic_string& insert(size_type __pos1, const basic_string& __str, size_type 
__pos2, size_type __n=npos);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1033:19:
 note: candidate function not viable: no known conversion from 'int' to 'const 
value_type *' (aka 'const char *') for 2nd argument
    basic_string& insert(size_type __pos, const value_type* __s, size_type __n);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1028:13:
 note: candidate template ignored: disabled by 'enable_if' [with _Tp = int]
            __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
            ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1050:13:
 note: candidate template ignored: disabled by 'enable_if' [with 
_ForwardIterator = int]
            __is_forward_iterator<_ForwardIterator>::value
            ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1022:19:
 note: candidate function not viable: requires 2 arguments, but 3 were provided
    basic_string& insert(size_type __pos1, const basic_string& __str);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1024:19:
 note: candidate function not viable: requires 2 arguments, but 3 were provided
    basic_string& insert(size_type __pos1, __self_view __sv) { return 
insert(__pos1, __sv.data(), __sv.size()); }
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1034:19:
 note: candidate function not viable: requires 2 arguments, but 3 were provided
    basic_string& insert(size_type __pos, const value_type* __s);
                  ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/string:1036:19:
 note: candidate function not viable: requires 2 arguments, but 3 were provided
    iterator      insert(const_iterator __pos, value_type __c);
                  ^