handle non-shallow clones
handle non-shallow clones

For local clones and clones by travis of repos with less than 50 commits `git fetch --unshallow` was throwing a fatal error.

Also, it's probably a little cleaner to do repo manipulation stuff in the `before_install` section.
zhmz90/DataFrames.jl,scls19fr/DataFrames.jl,pwoolcoc/DataFrames.jl,tshort/DataFrames.jl,JuliaPackageMirrors/DataFrames.jl,JuliaStats/DataFrames.jl,campanja-forks/DataFrames.jl,maximerischard/DataFrames.jl,maximerischard/DataFrames.jl,tkelman/DataFrames.jl,armgong/DataFrames.jl,emilbayes/DataFrames.jl,anriseth/DataFrames.jl,niloynibhochaudhury/DataFrames.jl,ccphillippi/DataFrames.jl