Bump template-haskell version to 2.15.0.0

Authored by RyanGlScott on Oct 28 2018, 9:25 PM.

Description

Bump template-haskell version to 2.15.0.0

Summary:
Commit 512eeb9bb9a81e915bfab25ca16bc87c62252064
(More explicit foralls (GHC Proposal 0007)) introduced breaking
changes to the Template Haskell AST. As a consequence of this, there
are libraries in the wild that now fail to build on GHC HEAD (for
instance, th-abstraction).

This properly bumps the template-haskell library's version number
to 2.15.0.0 so that these libraries can guard against these changes
using MIN_VERSION_template_haskell.

Test Plan: ./validate

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: Trac #15818

Differential Revision: https://phabricator.haskell.org/D5272