univalue: avoid warning: array subscript is of type 'char'

To avoid on:
g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix

the following warnings:
  CXX      univalue/univalue_write.o
univalue/univalue_write.cpp:33:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['"'] = "\\\"";
           ^~~~
univalue/univalue_write.cpp:34:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['\\'] = "\\\\";
           ^~~~~
univalue/univalue_write.cpp:35:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['/'] = "\\/";
           ^~~~
univalue/univalue_write.cpp:36:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['\b'] = "\\b";
           ^~~~~
univalue/univalue_write.cpp:37:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['\f'] = "\\f";
           ^~~~~
univalue/univalue_write.cpp:38:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['\n'] = "\\n";
           ^~~~~
univalue/univalue_write.cpp:39:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['\r'] = "\\r";
           ^~~~~
univalue/univalue_write.cpp:40:12: warning: array subscript is of type 'char' [-Wchar-subscripts]
    escapes['\t'] = "\\t";
           ^~~~~
8 warnings generated.