Fix to fasttree.rb formula

brew install fasttree was failing with:

    stevemoss@Steves-MacBook-Pro ~/D/P/R/R/u/mafft> brew install fasttree
    ==> Downloading http://www.microbesonline.org/fasttree/FastTree-2.1.7.c
    ######################################################################## 100.0%
    ==> llvm-gcc -lm -O3 -finline-functions -funroll-loops -Wall -o FastTree FastTree-.c
    i686-apple-darwin11-llvm-gcc-4.2: FastTree-.c: No such file or directory
    
    READ THIS: https://github.com/mxcl/homebrew/wiki/troubleshooting

Note the missing version numbers in the filename for the compilation step. This was due to an #{@version} in the fasttree.rb file, rather than #{version}.