Fixed bootstrap import error during less build process

Fixed `less:build` error with following message:
````
Running "less:build" (less) task
>> FileError: '../../node_modules/bootstrap/less/bootstrap.less' wasn't found in src/less/main.less on line 10, column 1:
>> 9  /
>> 10 @import '../../node_modules/bootstrap/less/bootstrap.less';
>> 11 /*
Warning: Error compiling src/less/main.less Use --force to continue.
````