Upgrade to GCC 4.8.x and replace PPL with ISL

The Travis-CI build runs out of memory when compiling PPL's C++ source (systematically when using 4 parallel jobs, intermittently when using 3 parallel jobs). GCC 4.8.x and above use the C-based ISL instead of PPL. This should require less memory to build and enable us to run 3+ parallel jobs.
Merge branch 'master' of github.com:spacecodesign/crosstool-configs