Upgrade minimum required compilers for MESOS

As discussed in the last community meeting we would like to upgrade the minimum mesos compiler version to GCC 4.8+, Clang 3.5. GCC primarily for Linux. Clang for OS X, as well as linux for enabling Mesos tooling improvements (clang-format, clang-tidy among others).

Some documents for reference:
Compilers by Distribution Version
Shows we can get GCC 4.8+ or clang 3.5+ on all supported platforms.

C++11 features supported by each compiler:
https://gcc.gnu.org/projects/cxx0x.html
http://clang.llvm.org/cxx_status.html