Allow versions-6.*

Note that `versions` uses SemVer rather than the PVP, and as a result, it
sometimes bumps the secondary version number even for non-breaking changes. As
a result, I am guarding against the major version number rather than the
secondary one.

llvm-pretty-bc-parser.cabal

@@ -135,7 +135,7 @@ Test-suite disasm-test
                        tasty-hunit,
                        tasty-sugar >= 2.2 && < 2.3,
                        text,
-                       versions < 6,
+                       versions < 7,
                        llvm-pretty,
                        llvm-pretty-bc-parser