sphinx-pre-install: use a requirements file

Instead of using 3 commands to install a virtualenv, use
a single one, reading the requirements from this file:

Documentation/sphinx/requirements.txt

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/sphinx/requirements.txt

···
+docutils==0.12
+Sphinx==1.4.9
+sphinx_rtd_theme

scripts/sphinx-pre-install

···
 # GNU General Public License for more details.
 
 my $virtenv_dir = "sphinx_1.4";
+my $requirement_file = "Documentation/sphinx/requirements.txt";
 
 #
 # Static vars
···
 
 			printf "\t$virtualenv $virtenv_dir\n";
 			printf "\t. $activate\n";
-			printf "\tpip install 'docutils==0.12'\n";
-			printf "\tpip install 'Sphinx==1.4.9'\n";
-			printf "\tpip install sphinx_rtd_theme\n";
+			printf "\tpip install -r $requirement_file\n";
 			$need++;
 		}
 	}