Speedup loading on OSX 

`using NetCDF` is pretty slow on OSX : 
```
julia> tic(); using NetCDF; toc()
elapsed time: 2.17279259 seconds
2.17279259
```

Most of this time is spend loading Homebrew, which in turn loads BinDeps. This PR gives me
```
julia> tic(); using NetCDF; toc()
elapsed time: 0.88669107 seconds
0.88669107
```
Which is a good improvement.

This may however lead to some kind of weird issue if the Homebrew.jl API changes someday.