Travis notes

Problem: travis machines use Ubuntu Trusty (14.04). This version does not contain some required packages, so it is needed to add it manually:

• ppa:ubuntu-toolchain-r/test: required for gcc-5, g++-5, gcc-5-multilib, g++-5-multilib...

• ppa:tes/ams: required for libftdi1-dev

• ppa:adrozdoff/mingw: required for i386-mingw32-gcc-5, i386-mingw32-g++-5, x86-w64-mingw32-gcc-5 and x86-w64-mingw32-g++-5. Surprisingly, arachne-pnr and yosys run twice as fast using gcc-5 instead of gcc-4.8.

Waiting for Ubuntu Xenial (16.04) support in travis...