Explicit import of s() is needed in not-so-old mgcv

NAMESPACE upgrade (3dc3111, PR #28 in github) broke mgcv < 1.8-0:

Error in eval(expr, envir, enclos) : could not find function "s"
Execution halted

This can be fixed in two ways: adding dependence on mgcv (>= 1.8-0)
in DESCRIPTION, or with explicit import of s in NAMESPACE. mgcv
1.8-0 was released on 25-Jun-2014 so that it is pretty young, and
therefore this fix suggests editing NAMESPACE.
Merge pull request #32 from jarioksa/old-mgcv-import-fix

Explicit import of s() is needed in not-so-old mgcv