Enable support for AVX instructions for Jets.

- Eigen versions < 3.3 only supported SIMD instructions that required
  16-byte alignment (SSE), whereas Eigen >= 3.3 also supports AVX
  instructions which require 32+-byte alignment.
- Previously we only ever requested 16-byte alignment for Jets (if >=
  C++11 was enabled) which resulted in Eigen assertions being triggered
  on some compilers as reported as Issue #251:
  https://github.com/ceres-solver/ceres-solver/issues/251.
- Now we use Eigen’s EIGEN_MAX_ALIGN_BYTES macro, defined in Eigen >=
  3.3 to specify the byte alignment for Jets when C++11 is enabled for
  Eigen >= 3.3.  For Eigen versions < 3.3, we maintain the previous
  behaviour of 16.

Change-Id: I749af7a70ae794e0c2a59301128db781e338422c