Bump to gcc 4.8 or newer, preferably gcc 4.9

[suntzu86]

We currently use gcc 4.7.3 for MOE/C++ stuff; this choice was kind of arbitrary and made just b/c that's what m/fsaric used and I borrowed it from them.

Unfortunately gcc 4.7.3's support for C++11 is incomplete; plus newer gccs include some potentially useful optimizations (esp if we go to Eigen...). So let's switch! We should at least bump to 4.8 if not 4.9 (omg, colored compiler messages).

It could be nice to do this right before or soon after open sourcing so that relatively few people are affected.

[gokceneraslan]

MOE compiles and works fine even with GCC 6.1.1 and Boost 1.60, so no reason to stick to older gcc versions.