Install solr extension from source

[stronk7]

Using 2.4.0 release + custom patch (from macports project) seems to
be the only working combination right now. Read the comments in the code
and the links there.

Resolves solr issue in #16, waiting for the upstream as described in #19

[stronk7]

For the records:

$ bin/moodle-docker-compose exec webserver vendor/bin/phpunit search_solr_engine_testcase
Moodle 3.8dev (Build: 20190520), a866c5c23866fe527282429384ddbcb902c41c14
Php: 7.3.5, pgsql: 9.6.7, OS: Linux 4.9.125-linuxkit x86_64
PHPUnit 7.5.9 by Sebastian Bergmann and contributors.

.............................                                     29 / 29 (100%)

Time: 32.75 seconds, Memory: 71.00 MB

OK (29 tests, 235 assertions)

[stronk7]

(and confirmed that search_solr_engine_testcase continue passing locally with the 2nd commit, fixing the patch version, good idea @andrewnicols )

$ bin/moodle-docker-compose exec webserver vendor/bin/phpunit search_solr_engine_testcase
Moodle 3.8dev (Build: 20190520), a866c5c23866fe527282429384ddbcb902c41c14
Php: 7.3.5, pgsql: 9.6.7, OS: Linux 4.9.125-linuxkit x86_64
PHPUnit 7.5.9 by Sebastian Bergmann and contributors.

.............................                                     29 / 29 (100%)

Time: 41.74 seconds, Memory: 71.00 MB

OK (29 tests, 235 assertions)