message: Update README.md for gcc version
diff: @@ -13,7 +13,7 @@ For Ginkgo core library:
  * _cmake 3.1+_
  * C++11 compliant compiler, one of:
  - * _gcc 4.8.1+_
  + * _gcc 5.4.0+_
  * _clang 3.3+_ (__TODO__: verify, works with 5.0)
  The Ginkgo GPU module has the following __additional__ requirements:
  @@ -41,7 +41,7 @@ For Ginkgo core library:
  * _cmake 3.1+_
  * C++11 compliant compiler, one of:
  - * _gcc 4.8.1+_ (__TODO__: verify)
  + * _gcc 5.4.0+_ (__TODO__: verify)
  * _clang 3.3+_ (__TODO__: verify)
  * _Apple LLVM 8.0+_ (__TODO__: verify)
  