Fix front-page claim about NumPy binary dependency

Because `sgp4` has also turned into a binary dependency.

Author: brandon-rhodes

documentation/index.rst

@@ -18,11 +18,9 @@ with the positions generated by the United States Naval Observatory
 and their *Astronomical Almanac*
 to within 0.0005 arcseconds (half a “mas” or milliarcsecond).
 
-* Written in pure Python and installs without any compilation.
-* Supports Python 2.6, Python 2.7 and Python 3.
-* Has NumPy as its only binary dependency,
-  the fundamental package for scientific computing with Python,
-  whose vector operations make Skyfield efficient.
+* Written in pure Python.
+* Installs without any compilation.
+* Supports Python 2.7 and Python 3.
 
 Computing the position of Mars in the sky is as easy as: