Support for Ruby 2.4 and 2D interpolation functions.

This release adds the following enhancements and fixes:

• Enhancements
  • Support for assigning Ruby Complex types to GSL::Matrix::Complex. (@v0dro)
  • Support for 2D interpolation by exposing GSLs interp2d and spline2d. (@v0dro)
  • Support for Ruby 2.4. (@badlamer)
• Fixes
  • Fix NArray superclass mismatch problem. (@v0dro)

Compile fixes with nmatrix on ruby 2.3.0

Gem was not compiling on Ruby 2.3.0 due to non-availability of correct headers. This minor release fixes that problem.

Compatibility with nmatrix and GSL 2.1

• Add compatibility with NMatrix data types.
• Add compatibility with GSL versions upto 2.1.