Use libpython base name only in final dlopen

This patch strips the extension from the libpython shared library name before the final dlopen, allowing the dynamic linker to be greedy. Without this, I get the following error on Ubuntu 13.04 (system python):

```
using PyPlot; plot(rand(10))
could not load module python: python: cannot open shared object file: No such file or directory
at /home/isaiah/.julia/PyPlot/src/PyPlot.jl:32
at In[2]:1
 in pyinitialize at /home/isaiah/.julia/PyCall/src/PyCall.jl:365
 in pyimport at /home/isaiah/.julia/PyCall/src/PyCall.jl:105
 in include at boot.jl:238
```
For background: `libpython2.7.so` does not exist as a symlink or otherwise in the standard search paths. It only exists as the symlink `/usr/lib/python2.7/config-x86_64-linux-gnu/libpython2.7.so` -> `/usr/lib/x86_64-linux-gnu/libpython2.7.so.1`. With this patch, the dynamic linker picks up the latter.