Disable unit tests that use private resources

[dongyoungy]

This is an attempt to enable automatic unit tests on external pull requests via CircleCI.
This PR:

1. disable unit tests using our private resources via @Ignore annotation
2. comment out git pull on our private resources in the CircleCI configuration

Please note that:

1. Currently our Redshift test instance is down, so CircleCI tests will fail
2. There has been issue with CircleCI image that we use for our jdk7 tests: https://discuss.circleci.com/t/apt-get-update-return-404/29237/8
   I have included a temporary fix to it, but we may need to use a different image for a proper fix.

[pyongjoo]

I think it looks good. Let me disable the private repo itself and let you know. In this way, we can fundamentally guarantee no access to the repo.

[pyongjoo]

@dongyoungy Currently, the errors are due to our stopped Redshift cluster. Can you also simply deactivate those unit tests?

[pyongjoo]

@dongyoungy Can you fix the error and merge this to the master? Not urgent, but I would like to base my branch on this.

[codecov-io]

Codecov Report

Merging #366 into master will decrease coverage by 2.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage    70.9%   68.64%   -2.25%     
==========================================
  Files         168      168              
  Lines       11517    11517              
  Branches     1879     1879              
==========================================
- Hits         8165     7905     -260     
- Misses       2868     3106     +238     
- Partials      484      506      +22

Impacted Files	Coverage Δ
...n/java/org/verdictdb/sqlsyntax/RedshiftSyntax.java	5.56% <0%> (-72.22%)	⬇️
...erdictdb/core/scrambling/ScramblingMethodBase.java	57.9% <0%> (-31.57%)	⬇️
...rdictdb/core/querying/CreateTableAsSelectNode.java	65.52% <0%> (-24.13%)	⬇️
...ava/org/verdictdb/metastore/ScrambleMetaStore.java	67.23% <0%> (-23.88%)	⬇️
...ictdb/core/scrambling/UniformScramblingMethod.java	70.32% <0%> (-21.87%)	⬇️
...org/verdictdb/sqlreader/ScrambleTableReplacer.java	65.56% <0%> (-16.66%)	⬇️
...ain/java/org/verdictdb/commons/StringSplitter.java	82.76% <0%> (-13.79%)	⬇️
...va/org/verdictdb/coordinator/ExecutionContext.java	71.67% <0%> (-13.35%)	⬇️
src/main/java/org/verdictdb/sqlreader/CondGen.java	66.67% <0%> (-8.77%)	⬇️
...in/java/org/verdictdb/sqlreader/ExpressionGen.java	75.31% <0%> (-7.22%)	⬇️
... and 17 more

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[dongyoungy]

I have further disabled unit tests with redshift and since all unit tests get passed, I am merging this PR.