using pypi to track xgboost dependency

.gitmodules

 [submodule "alphamind/pfopt"]
 	path = alphamind/pfopt
 	url = https://github.com/alpha-miner/portfolio-optimizer.git
-[submodule "xgboost"]
-	path = xgboost
-	url = https://github.com/dmlc/xgboost.git

requirements.txt

@@ -3,12 +3,13 @@ cython >= 0.25.2
 cvxpy >= 1.0.3
 deprecated >= 1.1.0
 ecos >= 2.0.4
+finance-python >= 0.5.7
+numba >= 0.33.0
 numpy >= 1.12.1
 pandas >= 0.19.2
+psycopg2 >= 2.7.1
 scikit-learn >= 0.18.1
-numba >= 0.33.0
 scipy >= 0.19.0
 simpleutils >= 0.1.2
 sqlalchemy >= 1.1.14
-psycopg2 >= 2.7.1
-finance-python >= 0.5.7
\ No newline at end of file
+xgboost >= 0.90.0
\ No newline at end of file

xgboost @ 680a1b36

-Subproject commit 680a1b36f3d951f5a14358db236670c259fa2839