Try a fix for py27 build

Makefile

@@ -16,5 +16,5 @@ coverage:
 	nosetests skimage --with-coverage --cover-package=skimage
 
 html:
-	pip install -q sphinx
+	pip install -q sphinx==1.2.3
 	export SPHINXOPTS=-W; make -C doc html

tools/travis_script.sh

@@ -10,6 +10,7 @@ section_end "Test.with.min.requirements"
 section "Build.docs"
 if [[ ($PY != 2.6) && ($PY != 3.2) ]]; then
     sudo apt-get install -qq texlive texlive-latex-extra dvipng
+    pip install -U setuptools pip
     make html
 fi
 section_end "Build.docs"