Remove kernel version assertions

Summary: It doesn't make any sense to assert on the contents of a string that
some user typed into their custom kernel configuration file. Assertions
are for developers, not for users.

Anyway, why would you assert that the minor version is non-zero? Some day there will be a 4.0.
Closes https://github.com/facebook/hhvm/pull/4258

Reviewed By: @paulbiss

Differential Revision: D1685143

Signature: t1:1685143:1416292812:b11cdc712802fb03bdb702447dbc252ed2fbf4e8