Fix sys.path manipulation in pavement.py, with detailed comment

Restore reverted changes from paver part 1

scripts/create-dev-env.sh

@@ -482,7 +482,7 @@ pip install -r $BASE/edx-platform/requirements/edx/pre.txt
 output "Installing edX requirements"
 # Install prereqs
 cd $BASE/edx-platform
-rake install_prereqs
+paver install_prereqs
 
 # Final dependecy
 output "Finishing Touches"
@@ -490,7 +490,7 @@ cd $BASE
 pip install argcomplete
 cd $BASE/edx-platform
 bundle install
-rake install_prereqs
+paver install_prereqs
 
 mkdir -p "$BASE/log"
 mkdir -p "$BASE/db"
@@ -523,7 +523,7 @@ if [[ ! $quiet ]]; then
 
    To start the Django on port 8000
 
-        $ rake lms
+        $ paver lms
 
    Or to start Django on a different <port#>