CLN: start using numpy-1.7 API
Now that numpy-1.6 support is dropped we could proceed to use np-1.7 API.

This involves fixing a macro or two deep down in pandas internals, but otherwise should be pretty straightforward. As a bonus we can get rid of these annoying build warnings:

```
gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -Ipandas/src/klib -Ipandas/src -I/home/immerrr/.conda/envs/pandas/lib/python2.7/site-packages/numpy/core/include -I/home/immerrr/.conda/envs/pandas/include/python2.7 -c pandas/index.c -o build/temp.linux-x86_64-2.7/pandas/index.o
In file included from /home/immerrr/.conda/envs/pandas/lib/python2.7/site-packages/numpy/core/include/numpy/ndarraytypes.h:1761:0,
                 from /home/immerrr/.conda/envs/pandas/lib/python2.7/site-packages/numpy/core/include/numpy/ndarrayobject.h:17,
                 from /home/immerrr/.conda/envs/pandas/lib/python2.7/site-packages/numpy/core/include/numpy/arrayobject.h:4,
                 from pandas/index.c:244:
/home/immerrr/.conda/envs/pandas/lib/python2.7/site-packages/numpy/core/include/numpy/npy_1_7_deprecated_api.h:15:2: warning: #warning "Using deprecated NumPy API, disable it by " "#defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp]
```
xref https://github.com/pydata/numexpr/pull/228/files
