ci(.travis.yml): use bzip2 instead of gzip for compressing Robot Framework report.

bzip2 produces less output and makes report lighter:

$ cat target/robotframework-reports/output.xml | gzip  | base64 | wc -l
1662
$ cat target/robotframework-reports/output.xml | bzip2 | base64 | wc -l
1023

Author: php-coder

.travis.yml

@@ -40,10 +40,16 @@ script:
 
 after_failure:
   # In order to be able debug robot framework test flakes we need to have a report.
-  # Just encode it to a gzipped binary form and dump to console.
+  # We encode it to a bzipped binary form and dump to console. It can be
+  # examined later with the following commands:
+  #
+  # $ curl -s https://api.travis-ci.org/v3/job/$JOB_ID/log.txt | sed -n '/^===== REPORT START/,/^===== REPORT END/p' | sed -e 1d -e '$d' -e 's/\r$//' | base64 -d | gzip -dc >target/robotframework-reports/output.xml
+  # $ ./mvnw robotframework:rebot
+  # $ open target/robotframework-reports/report.html
+  #
   - if fgrep -qs 'status="FAIL"' target/robotframework-reports/output.xml; then
       echo '===== REPORT START =====';
-      cat target/robotframework-reports/output.xml | gzip -c | base64;
+      bzip2 <target/robotframework-reports/output.xml | base64;
       echo '===== REPORT END =====';
       sleep 5;
     fi