Don't install nltk in editable mode

requirements/edx-sandbox/base.txt

@@ -11,4 +11,4 @@ pyparsing==2.0.1
 matplotlib==1.3.1
 
 # We forked NLTK just to make it work with setuptools instead of distribute
--e git+https://github.com/edx/nltk.git@2.0.6#egg=nltk
+git+https://github.com/edx/nltk.git@2.0.6#egg=nltk==2.0.6

requirements/edx/github.txt

@@ -13,7 +13,7 @@
 -e git+https://github.com/edx/django-wiki.git@cd0b2b31997afccde519fe5b3365e61a9edb143f#egg=django-wiki
 -e git+https://github.com/edx/django-oauth2-provider.git@0.2.7-fork-edx-5#egg=django-oauth2-provider
 -e git+https://github.com/edx/MongoDBProxy.git@25b99097615bda06bd7cdfe5669ed80dc2a7fed0#egg=mongodb_proxy
--e git+https://github.com/edx/nltk.git@2.0.6#egg=nltk
+git+https://github.com/edx/nltk.git@2.0.6#egg=nltk==2.0.6
 -e git+https://github.com/dementrock/pystache_custom.git@776973740bdaad83a3b029f96e415a7d1e8bec2f#egg=pystache_custom-dev
 -e git+https://github.com/eventbrite/zendesk.git@d53fe0e81b623f084e91776bcf6369f8b7b63879#egg=zendesk
 -e git+https://github.com/un33k/django-ipware.git@42cb1bb1dc680a60c6452e8bb2b843c2a0382c90#egg=django-ipware