Prevent intermittent MatchError in SelfDescValidationSpec by running tests sequentially

[fblundun]

Found by running the test suite on a loop until it failed.

[error] ! validating a correct self-desc JSON should return the JSON in a Success
[error]   MatchError: null (Resolver.scala:282)
[error] com.snowplowanalytics.iglu.client.Resolver.unsafeLookupSchema(Resolver.scala:282)
[error] com.snowplowanalytics.iglu.client.validation.ValidatableJsonMethods$.getSelfDescribingSchema(validatableJson.scala:182)
[error] com.snowplowanalytics.iglu.client.validation.ValidatableJsonMethods$.validateAsSelfDescribing(validatableJson.scala:197)
[error] com.snowplowanalytics.iglu.client.validation.ValidatableJsonMethods$.validate(validatableJson.scala:109)
[error] com.snowplowanalytics.iglu.client.validation.ValidatableJsonNode.validate(validatableJson.scala:238)
[error] com.snowplowanalytics.iglu.client.validation.SelfDescValidationSpec$$anonfun$e1$1.apply(SelfDescValidationSpec.scala:50)
[error] com.snowplowanalytics.iglu.client.validation.SelfDescValidationSpec$$anonfun$e1$1.apply(SelfDescValidationSpec.scala:50)
[error] com.snowplowanalytics.iglu.client.validation.SelfDescValidationSpec.e1(SelfDescValidationSpec.scala:50)
[error] com.snowplowanalytics.iglu.client.validation.SelfDescValidationSpec$$anonfun$is$1$$anonfun$apply$1$$anonfun$apply$2$$anonfun$apply$3$$anonfun$apply$4$$anonfun$apply$5$$anonfun$apply$6$$anonfun$apply$7$$anonfun$apply$8$$anonfun$apply$9$$anonfun$apply$11.apply(SelfDescValidationSpec.scala:32)
[error] com.snowplowanalytics.iglu.client.validation.SelfDescValidationSpec$$anonfun$is$1$$anonfun$apply$1$$anonfun$apply$2$$anonfun$apply$3$$anonfun$apply$4$$anonfun$apply$5$$anonfun$apply$6$$anonfun$apply$7$$anonfun$apply$8$$anonfun$apply$9$$anonfun$apply$11.apply(SelfDescValidationSpec.scala:32)

[fblundun]

Apparently caused by cache.get returning Some(null) here.

Probably related to #36

[fblundun]

This seems to be solved by adding

    testOptions in Test += Tests.Argument("sequential"),

as described in this specs2 issue: etorreborre/specs2#60