Workaround weird build issue on manylinux docker image (centos 5)

[ofrei]

The -lrt flag must be the last in linker command, that's why I'm adding it into target_link_libary.
I've tried to add into CMAKE_EXE_LINKER_FLAGS, but then it is prepended somewhere in the middle of link command, and has no effect.

Without hacking around librt I used to get these errors:

[ 98%] Linking CXX executable ../../bin/bigartm
/usr/local/lib/libboost_thread.a(thread.o): In function `boost::this_thread::no_interruption_point::hiden::sleep_until(timespec const&)':
thread.cpp:(.text+0x41c): undefined reference to `clock_gettime'
thread.cpp:(.text+0x4a1): undefined reference to `clock_gettime'
/usr/local/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_for(timespec const&)':
thread.cpp:(.text+0x13de): undefined reference to `clock_gettime'
collect2: error: ld returned 1 exit status
make[2]: *** [bin/bigartm] Error 1
make[1]: *** [src/bigartm/CMakeFiles/bigartm.dir/all] Error 2
make: *** [all] Error 2

And, for `libartm.so``, this:

root@08bc0b3dbc30:/usr/local/lib/python2.7/dist-packages/artm# python
Python 2.7.3 (default, Oct 26 2016, 21:01:49)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> ctypes.CDLL('/usr/local/lib/python2.7/dist-packages/artm/libartm.so')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/ctypes/__init__.py", line 365, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: /usr/local/lib/python2.7/dist-packages/artm/libartm.so: undefined symbol: clock_gettime

[ofrei]

Keep this PR open because it is related to pip install feature - which has highest priority among all users requests to BigARTM.