Merge tag 'v0.3.0' into develop

tag version 0.3.0
Fix the wrong size of the array of gradients

 - s_gradients array was specified of size 12, and used with a modulus 12,
   but is only filled with 8 gradient vectors !
 => if the rest of the array is filled with zeros,
    this degrades the variety of the noise produced

 - I believe the error comes from having converted a 3D implementation,
   where 12 gradients are used (for standard implementation)
 - And by the way, using a modulus 8 is much more optimzed by the compiler :)
fix typo in ChangeLog
Merge branch 'fix-simplex-wrong-size-of-gradient-array' of https://github.com/SRombauts/mapmaker into hotfix-simplex-wrong-size-of-gradient-array
Merge branch 'hotfix-simplex-wrong-size-of-gradient-array' into develop
modify ChangeLog for the previous commit
do not use 'using' for old compilers
do not use 'emplace' for old compilers
add soversion for the library
bump version to 0.3.1
Merge branch 'release-0.3.1'