add dependency on g++ for foreman-compute

otherwise unf_ext can't build it's native extension:

```
Installing unf_ext (0.0.6) with native extensions 
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.9.1 extconf.rb 
checking for main() in -lstdc++... no
checking for ruby/encoding.h... yes
creating Makefile

make
compiling unf.cc
make: g++: Command not found
make: *** [unf.o] Error 127
```