(folly osx) test for the zip, not the dir

Summary: Apparently `configure` creates this subdirectory and then the curl and unzip commands don't run, and then `make check` fails. Instead, do it if the zip file doesn't exist.

Test Plan:
git clone https://github.com/facebook/folly.git
cd folly/folly
/path/to/this/bootstrap-osx-homebrew.sh
make
make check

Reviewed By: andrei.alexandrescu@fb.com

Subscribers: exa, folly-diffs@, yfeldblum, davejwatson

FB internal diff: D1882472

Signature: t1:1882472:1425920470:bdab25967d3b8e52c690d5627168077c65829b92

folly/bootstrap-osx-homebrew.sh

@@ -37,7 +37,7 @@ autoreconf -i
 ./configure CPPFLAGS=-I"$DOUBLE_CONVERSION_HOME/src" LDFLAGS=-L"$DOUBLE_CONVERSION_HOME"
 
 pushd test
-test -d gtest-1.7.0 || {
+test -e gtest-1.7.0.zip || {
     curl -O https://googletest.googlecode.com/files/gtest-1.7.0.zip
     unzip gtest-1.7.0.zip
 }