Composer, apache/thrift keep lib/php only

[dbaltas]

This PR aims to reduce disk usage.
apache/thrift comes with the whole C++ implementation of the software, plus with libraries for ~25 programming languages.

By removing the unnecessary files and folders, we end up with 560K of dependencies instead of 19MB.

This issue becomes more of a concern to projects that commit the vendor/ directory.

master

9:25:07 ➜  opencensus-php-exporter-jaeger git:(master)  rm -rf vendor
9:25:12 ➜  opencensus-php-exporter-jaeger git:(master)  composer install
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
### more composer output here
Generating autoload files
9:25:19 ➜  opencensus-php-exporter-jaeger git:(master)  du -hs vendor/apache/thrift
 19M	vendor/apache/thrift

composer-thrift-keep-php-lib-only

9:25:34 ➜  opencensus-php-exporter-jaeger git:(composer-thrift-keep-php-lib-only)  rm -rf vendor
9:25:38 ➜  opencensus-php-exporter-jaeger git:(composer-thrift-keep-php-lib-only)  composer install
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
### more composer output here
> cd vendor/apache/thrift && ls | grep -v lib | xargs rm -rf
/tmp/opencensus-php-exporter-jaeger/vendor/apache/thrift
> cd vendor/apache/thrift/lib && ls | grep -v php | xargs rm -rf
/tmp/opencensus-php-exporter-jaeger/vendor/apache/thrift/lib
9:25:46 ➜  opencensus-php-exporter-jaeger git:(composer-thrift-keep-php-lib-only)  du -hs vendor/apache/thrift
560K	vendor/apache/thrift

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[dbaltas]

I signed it!

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[dbaltas]

Hmm, it seems that in circle-ci, the composer install command does not execute the composer script for post-install-cmd.
For example the php 7.2 build here should include the following commands in the output.

> cd vendor/apache/thrift && ls | grep -v lib | xargs rm -rf
/tmp/opencensus-php-exporter-jaeger/vendor/apache/thrift
> cd vendor/apache/thrift/lib && ls | grep -v php | xargs rm -rf
/tmp/opencensus-php-exporter-jaeger/vendor/apache/thrift/lib