[SPARK-4890] Ignore downloaded EC2 libs

[nchammas]

PR #3737 changed spark-ec2 to automatically download boto from PyPI. This PR tell git to ignore those downloaded library files.

[SparkQA]

Test build #24724 has started for PR 3770 at commit 7e22baa.

• This patch merges cleanly.

[nchammas]

cc @JoshRosen

[SparkQA]

Test build #24725 has started for PR 3770 at commit 5c440d3.

• This patch merges cleanly.

[SparkQA]

Test build #24724 has finished for PR 3770 at commit 7e22baa.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24724/
Test PASSed.

[SparkQA]

Test build #24725 has finished for PR 3770 at commit 5c440d3.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24725/
Test PASSed.

[JoshRosen]

LGTM, so I've merged this into master (1.3.0). Thanks!