Fix bash conditional

Bug: 29630077
Test: ./gradlew lint
Change-Id: I0be0f3a0035929bf4db27f4429e4ad3650ffd1ff
diff --git a/gradlew b/gradlew
index f942e0b..30aa96f 100755
--- a/gradlew
+++ b/gradlew
@@ -7,7 +7,7 @@
 ##############################################################################
 
 # Pick the correct fullsdk for this OS.
-if [ $os = "Darwin" ]; then
+if [ "$os" = "Darwin" ]; then
     plat="darwin"
 else
     plat="linux"