Revert update of Cabal spec for Travis CI

See #26

crypto-enigma.cabal

@@ -1,4 +1,3 @@
-cabal-version:          2.2
 name:                   crypto-enigma
 -- http://www.haskell.org/haskellwiki/Package_versioning_policy
 -- PVP summary:         +-+------- breaking API changes
@@ -25,7 +24,7 @@ description:            The crypto-enigma package is an Enigma machine simulator
                         appear garbled.)
 homepage:               https://github.com/orome/crypto-enigma-hs
 bug-reports:            https://github.com/orome/crypto-enigma-hs/issues
-license:                BSD-3-Clause
+license:                BSD3
 license-file:           LICENSE
 author:                 Roy Levien
 maintainer:             royl@aldaron.com
@@ -35,6 +34,7 @@ build-type:             Simple
 extra-source-files:     CHANGELOG.md
                         README.md
 extra-doc-files:        figs/*.jpg
+cabal-version:          1.22
 
 source-repository head
         type:           git