Update parslet dependency to a version that has more relaxed blankslate requirements

When integrating jekyll with a project that has dependencies which require a newer (>= 3.0) version of  blankslate there are conflicts with the 2.x version of blankslate that parslet 1.5.0 requires.

This small change updates parslet to a version that relaxed the blankslate requirement to >= 2.0 and <= 4.0.

kschiess/parslet@70e3138bc9c8fd663cf93bc69a4c2bb8b80f3852 is the important change in parslet that addresses the blankslate dependancy.

The test suite still passes with parslet 1.6.2.