updated build.gradle, AndroidManifest.xml, DemoActivity, SlidingUpPanelL...

In general:
- updated gradle wrapper version to most recent

In demo:
- changed *build.gradle* to support latest build tools;
- changed *minSdk* to 11 (as demo requires ActionBar and suppresses lint warnings on that account)
- generalised versions of dependencies ("+" instead explicit numbers where possible)
- removed *lint.xml* and *dimens.xml*, as they are not in any use
- removed `@SuppressLint` from DemoActivity, as it explicitly uses API 11 for now

In library:
- changed *build.gradle* (same as above)
- simplified return statements in SlidingUpPanelLayout