Compiling requires GCC >= 5.0

[tautschnig]

As of 3eab185 the code base no longer compiles with GCC 4.9, due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59987:

expr2java.cpp: In member function ‘virtual std::string expr2javat::convert_constant(const constant_exprt&, unsigned int&)’:
expr2java.cpp:285:15: error: ‘hexfloat’ is not a member of ‘std’
     buffer << std::hexfloat;
               ^
../common:214: recipe for target 'expr2java.o' failed

Either a workaround or an update of COMPILING.md are required.

[NlightNFotis]

@tautschnig Are there any consequences to just upgrading the compiler version requirement?

[tautschnig]

I think it's ok to upgrade.