Update bootstrap.sh

Following the third comment here (of dafis)  http://stackoverflow.com/questions/16128503/trouble-with-installing-cabal-on-mac it seems that mtl-1.* should not be permitted by the regex on line 56, 

    MTL_VER="2.1.2";       MTL_VER_REGEXP="[12]\."     # == 1.* || == 2.*

since in fact mtl-1 is rejected by the build-depends in the cabal file, which mentions  

    mtl      >= 2.0      && < 3

But perhaps this is a wrong understanding.
Merge pull request #1283 from michaelt/master

mtl-1.* wrongly permitted by boostrap.sh?