Fixed check method of $USE_GET_PIP

'[[ -n "$USE_GET_PIP" ]]' always TRUE
because $USE_GET_PIP is '0' or '1'.

Change-Id: I73c4c6befe2126882ef21991b2a3fe712b2ac388
Closes-Bug: #1259824
diff --git a/tools/install_pip.sh b/tools/install_pip.sh
index a65a77e..d714d33 100755
--- a/tools/install_pip.sh
+++ b/tools/install_pip.sh
@@ -87,7 +87,7 @@
 # Eradicate any and all system packages
 uninstall_package python-pip
 
-if [[ -n "$USE_GET_PIP" ]]; then
+if [[ "$USE_GET_PIP" == "1" ]]; then
     install_get_pip
 else
     install_pip_tarball