Changed from .pth to add2virtualenv equivalent

The mycroft-core.pth file wasn't addequate to get Python to fully
accept mycroft-core into the system path.  For example, some of
the integration tests would fail in odd ways.

So switched to using add2virtualenv, which is was fine.  But you
can't call that within the dev_setup.sh script since it is a
shell function.  So instead this code does essentially what
add2virtualenv does.

dev_setup.sh

@@ -46,7 +46,7 @@ fi
 
 # Check whether to build mimic (it takes a really long time!)
 build_mimic='y'
-if [[ "$1" == '-sm' ]] ; then 
+if [[ "$1" == '-sm' ]] ; then
   build_mimic='n'
 else
   # first, look for a build of mimic in the folder
@@ -79,15 +79,23 @@ easy_install pip==7.1.2 # force version of pip
 pip install --upgrade virtualenv
 
 # Add mycroft-core to the virtualenv path
-echo "$TOP" > ${VIRTUALENV_ROOT}/lib/python2.7/site-packages/mycroft-core.pth
+# (This is equivalent to typing 'add2virtualenv $TOP', except
+# you can't invoke that shell function from inside a script)
+if ! grep -q "mycroft-core" "${VIRTUALENV_ROOT}/lib/python2.7/site-packages/_virtualenv_path_extensions.pth"; then
+   echo "Adding mycroft-core to virtualenv path"
+   sed -i.tmp '1 a\
+'"$TOP"'
+' "${VIRTUALENV_ROOT}/lib/python2.7/site-packages/_virtualenv_path_extensions.pth"
+fi
+
 
 # install requirements (except pocketsphinx)
 # removing the pip2 explicit usage here for consistency with the above use.
-pip install -r requirements.txt 
+pip install -r requirements.txt
 
 if  [[ $(free|awk '/^Mem:/{print $2}') -lt  1572864 ]] ; then
   CORES=1
-else 
+else
   CORES=$(nproc)
 fi
 echo "Building with $CORES cores."