[Travis CI] add link to gcc-5 as gcc-[gcc version]
[Travis CI] add link to gcc-5 as gcc-[gcc version]

 - Boost bjam expects a gcc/g++ executable/s named gcc/g++-[gcc version],
   but the debian package exports the executable/s as "gcc/g++-5".
   Make a symlink to solve the problem.

Manu343726/siplasplas,GueimUCM/siplasplas,GueimUCM/siplasplas,Manu343726/siplasplas,GueimUCM/siplasplas,Manu343726/siplasplas,Manu343726/siplasplas