lib: raise the minimum Python version from 2.6 to 2.7
lib: raise the minimum Python version from 2.6 to 2.7

As discussed in #1811

PR-URL: https://github.com/nodejs/node-gyp/pull/1818
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: João Reis <5095b819f0ac7660978fa44706cfa411a9116ec5@janeasystems.com>

nodegit/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp