README: replace single quotes with double quotes

Commands may file in certain environments where single and double quotes cannot be used interchangeably.

Fix #390

README.rst

@@ -36,7 +36,7 @@ Install latest release version from PYPI:
 .. code:: bash
 
     # with SteamClient dependecies
-    pip install -U 'steam[client]'
+    pip install -U "steam[client]"
 
     # without (only when using parts that do no rely on gevent, and protobufs)
     pip install -U steam
@@ -46,12 +46,12 @@ Installing directly from ``github`` repository:
 .. code:: bash
 
     # cutting edge from master
-    pip install 'git+https://github.com/ValvePython/steam#egg=steam'
+    pip install "git+https://github.com/ValvePython/steam#egg=steam"
 
     # specific version tag (e.g. v1.0.0)
-    pip install 'git+https://github.com/ValvePython/[email protected]#egg=steam[client]'
+    pip install "git+https://github.com/ValvePython/[email protected]#egg=steam[client]"
     # without SteamClient extras
-    pip install 'git+https://github.com/ValvePython/[email protected]#egg=steam'
+    pip install "git+https://github.com/ValvePython/[email protected]#egg=steam"
 
 Vagrant
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