[travis] cleanup travis file

[gicmo]

• Use compiler directive instead of custom matrix+env variables
• Only install g++-4.8 for g++ builds
• Use update-alternatives to select g++-4.8

[coveralls]

Coverage remained the same at 86.25% when pulling 3edbcee on gicmo:travis-cleanup into bb69fee on G-Node:master.

[coveralls]

Coverage remained the same at 86.25% when pulling c3fe691 on gicmo:travis-cleanup into bb69fee on G-Node:master.