Bump sphinx to 1.5.1 to avoid doc compilation failure

There is conflict between recent docutils and old sphinx which
lead to build error for external images or SVG images
(see https://sourceforge.net/p/docutils/bugs/301/)
The fix is included into Sphinx 1.5.1

Change-Id: Iecddc67fe19279326827b5b76fb084b4e316c78d

requirements.txt

@@ -1,6 +1,6 @@
 oslosphinx>=2.5.0 # Apache-2.0
 rst2pdf
 six>=1.9.0
-sphinx!=1.2.0,!=1.3b1,<1.3,>=1.1.2
+sphinx>=1.5.1 # BSD  -- to avoid https://sourceforge.net/p/docutils/bugs/301/
 sphinxcontrib-httpdomain
 sphinx_rtd_theme