Cannot use PyPlot

Although I am able to install `PyPlot`, I get an error message when importing it into a Julia session:

```
julia> Pkg.update()
INFO: Initializing package repository /home/carlos/.julia.
INFO: Cloning METADATA from git://github.com/JuliaLang/METADATA.jl
INFO: Updating METADATA...
INFO: Computing changes...
INFO: No packages to install, update or remove.

julia> Pkg.add("PyPlot")
INFO: Cloning cache of PyPlot from git://github.com/stevengj/PyPlot.jl.git
INFO: Cloning cache of Color from git://github.com/JuliaLang/Color.jl.git
INFO: Cloning cache of PyCall from git://github.com/stevengj/PyCall.jl.git
INFO: Installing PyPlot v1.0.11
INFO: Installing Color v0.2.6
INFO: Installing PyCall v0.0.0
INFO: REQUIRE updated.

julia> using PyPlot
ERROR: PyError (PyImport_ImportModule)
AttributeError("'module' object has no attribute 'core'",)
File "/usr/local/lib/python2.7/dist-packages/matplotlib/__init__.py", line 129, in
from matplotlib.cbook import is_string_like
File "/usr/local/lib/python2.7/dist-packages/matplotlib/cbook.py", line 28, in
import numpy as np
File "/usr/local/lib/python2.7/dist-packages/numpy/__init__.py", line 137, in
import add_newdocs
File "/usr/local/lib/python2.7/dist-packages/numpy/add_newdocs.py", line 9, in
from numpy.lib import add_newdoc
File "/usr/local/lib/python2.7/dist-packages/numpy/lib/__init__.py", line 13, in
from polynomial import *
File "/usr/local/lib/python2.7/dist-packages/numpy/lib/polynomial.py", line 11, in
import numpy.core.numeric as NX

in pyerr_check at /home/carlos/.julia/PyCall/src/exception.jl:58
in pyimport at /home/carlos/.julia/PyCall/src/PyCall.jl:85
in include at boot.jl:238
in include_from_node1 at loading.jl:96
in reload_path at loading.jl:121
in require at loading.jl:50
at /home/carlos/.julia/PyPlot/src/PyPlot.jl:32

julia>
```

How can I get `PyPlot` up and running?

**I am running**:
- Linux Mint 13, KDE, 64 bit (based on Ubuntu 12.04 LTS)
- `python 2.7.3`
- `matplotlib 1.3.0`