Travis CI fails with Child module ... does not exist

Question

My git repository structure is as following:

-my-repo
  -.mvn
  -.travis.yml
  -my-parent-project
  -child-module
    -pom.xml
  -pom.xml

Where the parent pom includes

<modules>
    <module>child-module</module>
</modules>

And my .travis.yml looks like:

sudo: true
language: java
jdk: oraclejdk9
os: linux
before_install:
  - chmod +x ./my-project-parent/*
  - cd my-project-parent
install: true
script: ./mvnw clean install
cache:
  directories:
  - $HOME/.m2

When Travis CI build runs I'm getting:

[ERROR]   The project io.hvest:my-project-parent:1.0-SNAPSHOT (/home/travis/build/hVest/my-repo/my-project-parent/pom.xml) has 1 error
[ERROR]     Child module /home/travis/build/hVest/my-repo/my-project-parent/child-module of /home/travis/build/hVest/my-repo/my-project-parent/pom.xml does not exist

Locally maven runs smoothly, I don't understand why the build process running by Travis CI can't find the child-module, any ideas?