Unset VERILATOR_ROOT to fix build

Author: diadatp

.travis.yml

@@ -74,7 +74,7 @@ install:
  # Download, build and install Verilator 3.882.
  - export SYSTEMC_CXX_FLAGS=-pthread
  - bash install-verilator.sh
- - export VERILATOR_ROOT=${HOME}/verilator/share/verilator
+ # - export VERILATOR_ROOT=${HOME}/verilator/share/verilator
  - export PATH=${HOME}/verilator/bin:$PATH
  - verilator -V
  - ls -R ${VERILATOR_ROOT}