Added FlexibleContexts extension in Parser.hs to fix compilation.

ShellCheck/Parser.hs

@@ -17,7 +17,7 @@
     yOU should have received a copy of the GNU General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 -}
-{-# LANGUAGE NoMonomorphismRestriction, TemplateHaskell #-}
+{-# LANGUAGE NoMonomorphismRestriction, TemplateHaskell, FlexibleContexts #-}
 module ShellCheck.Parser (parseScript, runTests) where
 
 import ShellCheck.AST