TransformCaching: fix GC condition judgement error

Summary:
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**Summary**

<!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->
I am sending this PR to fix an error in TransformCaching module. See this issue [TransformCaching module try to collect cache every time I build bundle #46](https://github.com/facebook/metro-bundler/issues/46).

**Test plan**

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```
/**
   * When restarting packager we want to avoid running the collection over
   * again, so we store the last collection time in a file and we check that
   * first.
   */
  _collectCacheIfOldSync() {
    const {_rootPath} = this;
    const cacheCollectionFilePath = path.join(_rootPath, 'last_collected');
    const lastCollected = Number.parseInt(
      tryReadFileSync(cacheCollectionFilePath) || '',
      10,
    );
    if (
      Number.isInteger(lastCollected) &&
      Date.now() - lastCollected < GARBAGE_COLLECTION_PERIOD
    ) {
      return;
    }
    const effectiveCacheDirPath = path.join(_rootPath, CACHE_SUB_DIR);
    mkdirp.sync(effectiveCacheDirPath);
    collectCacheSync(effectiveCacheDirPath);
    fs.writeFileSync(cacheCollectionFilePath, Date.now().toString());
  }
```
Steps:
1. Create a new react-native project.
2. Put your breakpoint at this sentence `collectCacheSync(effectiveCacheDirPath);`.
3. Bundle the project, program will break at `collectCacheSync(effectiveCacheDirPath);`
4. Resume the program, finish the first time bundle.
5. Bundle again, this time program won't break.
Closes https://github.com/facebook/metro-bundler/pull/48

Differential Revision: D5726161

Pulled By: jeanlauliac

fbshipit-source-id: 0865f1bf25d6eb36f067ac3dc7764df9fd5026dc

packages/metro-bundler/src/lib/TransformCaching.js

@@ -272,7 +272,7 @@ class FileBasedCache {
     );
     if (
       Number.isInteger(lastCollected) &&
-      Date.now() - lastCollected > GARBAGE_COLLECTION_PERIOD
+      Date.now() - lastCollected < GARBAGE_COLLECTION_PERIOD
     ) {
       return;
     }