[Travis] Disabled symlinking of local dependency

[mnocon]

Example failure:
https://travis-ci.com/github/ezsystems/ezplatform-http-cache/jobs/486034188

Error:

 ---> ec18ce700090

Step 6/10 : COPY vendor/ezsystems/ezplatform-http-cache/docs/varnish/vcl/varnish5.vcl /etc/varnish/default.vcl

Service 'varnish' failed to build: COPY failed: stat /var/lib/docker/tmp/docker-builder696197820/var/www/ezplatform-http-cache/docs/varnish/vcl/varnish5.vcl: no such file or directory

Varnish container fails to builds, because it's not able to find the vendor/ezsystems/ezplatform-http-cache/docs/varnish/vcl/varnish5.vcl file.

This happens only for builds in ezplatform-http-cache repository and is caused by #648 .

In #648 I've changed the repository type to path. This has one unforseen consequence - packages downloaded using that method are symlinked by default (Ref: https://getcomposer.org/doc/05-repositories.md#path)

The PR dependency is moved on the host to /home/travis/build/ezplatform/ezplatform-http-cache and there is no ezsystems/ezplatform-http-cache in the vendor directory - there is only a symlink to the actual location.
Looks like Docker is unable to resolve (follow) the symlink when trying to copy the file and reports that the file does not exists.

This solution disables symliniking of the dependency -it's mirrored (copied) instead, so that the files are where they are supposed to be and everything works as before.

I've tested the solution on 3.2 with 2 PRs:
#649
ezsystems/ezplatform-http-cache#145
And Varnish container is setup correctly.