add missing instruction for Perl dependency

[monperrus]

Would have saved me 10 minutes.

Fix

Can't locate String/Interpolate.pm in @INC (you may need to install the String::Interpolate module) (@INC contains: /mnt/data/martin/defects4j/framework/lib /mnt/data/m
artin/defects4j/framework /mnt/data/martin/defects4j/framework/core /mnt/data/martin/defects4j/framework/core /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.30.0 /usr
/local/share/perl/5.30.0 /usr/lib/x86_64-linux-gnu/perl5/5.30 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.30 /usr/share/perl/5.30 /usr/local/lib/site_perl /usr/li
b/x86_64-linux-gnu/perl-base) at /mnt/data/martin/defects4j/framework/core/Utils.pm line 45.                                                                            
BEGIN failed--compilation aborted at /mnt/data/martin/defects4j/framework/core/Utils.pm line 45.                                                                        
Compilation failed in require at /mnt/data/martin/defects4j/framework/bin/defects4j line 34.                                                                            
BEGIN failed--compilation aborted at /mnt/data/martin/defects4j/framework/bin/defects4j line 34.