[Python] Set minimum NumPy version to 1.8.1.

Minimum set based on a regression in NumPy 1.8.0 which affects the SolutionArray
class.

This makes Trusty Tahr (14.04) the oldest supported version of Ubuntu (using the
Ubuntu-provided NumPy package).

Resolves #445.

SConstruct

@@ -1067,7 +1067,7 @@ env['python_cmd_esc'] = quoted(env['python_cmd'])
 
 # Python 2 Package Settings
 cython_min_version = LooseVersion('0.23')
-numpy_min_test_version = LooseVersion('1.6.1')
+numpy_min_test_version = LooseVersion('1.8.1')
 env['install_python2_action'] = ''
 if env['python_package'] == 'new':
     print ("WARNING: The 'new' option for the Python package is "

doc/sphinx/compiling/dependencies.rst

@@ -123,7 +123,8 @@ Optional Programs
 
   * Required to build the Cantera Python module, and to run significant portions
     of the test suite.
-  * Known to work with versions 1.7-1.11; Expected to work with version >= 1.4
+  * Known to work with versions 1.8.1-1.13. Expected to work with
+    versions >= 1.8.1.
 
 * `Cython <http://cython.org/>`_