docs: update requirement c++11 -> c++14

Author: ondovb

README.md

@@ -1,6 +1,6 @@
 Mash is normally distributed as a dependency-free binary for Linux or OSX (see
 https://github.com/marbl/Mash/releases). This source distribution is intended
-for other operating systems or for development. Mash requires c++11 to build,
-which is available in and GCC >= 4.8 and OSX >= 10.7.
+for other operating systems or for development. Mash requires c++14 to build,
+which is available in and GCC >= 5 and XCode >= 6.
 
 See http://mash.readthedocs.org for more information.