travis-ci: don't fail on 'example' instance stop

[Totktonada]

The 'example' instance in not installed by default in tarantool-2.4
since 2.4.1-66-g78a9f8698, in tarantool-2.5 since 2.5.0-133-g080beba06
and in all further tarantool versions; see 1 for more information.

[Totktonada]

From CI on tarantool-2.5:

[ERROR] Tests run: 17, Failures: 0, Errors: 1, Skipped: 3, Time elapsed: 0.534 s <<< FAILURE! - in org.tarantool.jdbc.JdbcTypesIT
[ERROR] testSetBigDecimalUsingNumber Time elapsed: 0.032 s <<< ERROR!
java.sql.SQLException: java.sql.SQLException: Failed to execute SQL: INSERT INTO test_types(KEY, F4) VALUES (?, ?), params: [0, 4.9E-324]
<...>
Caused by: java.sql.SQLException: Failed to execute SQL: INSERT INTO test_types(KEY, F4) VALUES (?, ?), params: [0, 4.9E-324]
<...>
Caused by: org.tarantool.TarantoolException: Type mismatch: can not convert 0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000049 to number

Does not look related to the solved problem. I guess it is due to 2.5.0-272-g30c57bb3e (the relevant issue):

Rules for numeric values are these:

1. Loss of significant digits (overflow) is an error.
2. Loss of insignificant digits is not an error.

I'll look at this later.

[coveralls]

Coverage remained the same at 77.66% when pulling 4688f44 on Totktonada/travis-ci-dont-fail-on-example-stop into 9936054 on master.