[Hot Fix] Remove 'bower_component' from caching list in travis cfg file
[Hot Fix] Remove 'bower_component' from caching list in travis cfg file

### What is this PR for?
After #2042 merged, cached `bower_components/` causes CI failure like
```
"message": "Unable to find suitable version for d3" (bower installation failure)
```
```
[ERROR] [{
[ERROR]   "code": "ECONFLICT",
[ERROR]   "name": "d3",
[ERROR]   "picks": [
[ERROR]     {
[ERROR]       "endpoint": {
[ERROR]         "name": "d3",
[ERROR]         "source": "d3",
[ERROR]         "target": "~3.3.13"
[ERROR]       },
[ERROR]       "canonicalDir": "/home/travis/build/apache/zeppelin/zeppelin-web/bower_components/d3",
[ERROR]       "pkgMeta": {
[ERROR]         "name": "d3",
[ERROR]         "version": "3.3.13",
[ERROR]         "main": "d3.js",
[ERROR]         "scripts": [
[ERROR]           "d3.js"
[ERROR]         ],
...
```

It can be resolved in each contributors' travis account by deleting all cache in their repository like [this](https://github.com/apache/zeppelin/pull/2042#issuecomment-284324008), but current Zeppelin master is under Apache travis account. So we need to clear the cache by removing `zeppelin-web/bower_components` in caching list in `.travis.yml`.

Let's see the master branch's CI turns to green light :)

### What type of PR is it?
Hot Fix

### What is the Jira issue?
No Jira issue for this

### How should this be tested?
No need to test

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: AhyoungRyu <fbdkdud93@hanmail.net>

Closes #2102 from AhyoungRyu/makeCIHappy and squashes the following commits:

05c802c [AhyoungRyu] Remove 'bower_component' from caching list in travis cfg file

(cherry picked from commit 77778e35ed563f8e3a71a9c647838ec951e8789a)
Signed-off-by: Mina Lee <bb9c4f68b7c04206772f92363630483486088f46@apache.org>

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