Try to fix the failing test

[tovbinm]

Related issues
Sometimes TravisCI fails to deserialize the model in test and fails with the following error:

com.salesforce.op.stages.base.sequence.SequenceEstimatorTest
  [+] SequenceEstimator[OPVector] should produce output feature (1 ms)
  [+] SequenceEstimator[OPVector] should copy (10 ms)
  [+] SequenceEstimator[OPVector] should be serializable (16 ms)
  [+] SequenceEstimator[OPVector] should fit a model (479 ms)
  [+] SequenceEstimator[OPVector] model should produce output feature (1 ms)
  [+] SequenceEstimator[OPVector] model should copy (20 ms)
  [+] SequenceEstimator[OPVector] model should be serializable (13 ms)
  [-] SequenceEstimator[OPVector] model should be json writable/readable (33 ms)
com.salesforce.op.stages.base.sequence.SequenceEstimatorTest > SequenceEstimator[OPVector] model should be json writable/readable FAILED
    org.scalatest.exceptions.TestFailedException: Failed to read the stage of type 'com.salesforce.op.stages.base.sequence.FractionOfResponsesModel'
        Caused by:
        java.lang.RuntimeException: Failed to read the stage of type 'com.salesforce.op.stages.base.sequence.FractionOfResponsesModel'
            Caused by:
            java.lang.RuntimeException: Failed to extract value for param 'counts' for an instance of type 'com.salesforce.op.stages.base.sequence.FractionOfResponsesModel' due to: Failed to parse argument 'counts' from value 'List(5, 4, 4)' of class 'scala.collection.immutable.$colon$colon'
                Caused by:
                java.lang.RuntimeException: Failed to parse argument 'counts' from value 'List(5, 4, 4)' of class 'scala.collection.immutable.$colon$colon'
                    Caused by:
                    org.json4s.package$MappingException: Parsed JSON values do not match with class constructor
                    args=
                    arg types=
                    constructor=public scala.Int()

Describe the proposed solution
Trying out different solutions...

Describe alternatives you've considered
NA

[codecov]

Codecov Report

Merging #356 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #356      +/-   ##
==========================================
+ Coverage   86.72%   86.73%   +0.01%     
==========================================
  Files         336      336              
  Lines       10799    10799              
  Branches      346      346              
==========================================
+ Hits         9365     9367       +2     
+ Misses       1434     1432       -2

Impacted Files	Coverage Δ
...es/src/main/scala/com/salesforce/op/OpParams.scala	89.79% <0%> (+4.08%)	⬆️

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