Bumping supported compiler versions for Stan 2.18

Announcements
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  • GCC 4.9.3+ (4.9.3 is the version that ships with RTools 3.4, required for rstan windows support).
  • Microsoft Visual Studio 2015 (as required by PyStan on Windows)
  • clang 3.4+

More details & up to date information: https://github.com/stan-dev/stan/wiki/Supported-C---Compilers-and-Language-Features

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This should probably also go under the Announcements tag.