Fix catch exception by reference

When compiling our codebase with clang we were getting the following
error:

GSL/tests/algorithm_tests.cpp:198:58: warning: catch handler catches by
value; should catch by reference instead
[misc-throw-by-value-catch-by-reference]
    CHECK_THROWS_AS(copy(src_span_dyn, dst_span_static), fail_fast);
                                                             ^
Looking at the catch source code exceptions were being caught by value.
One could have it designed so that users might say:

CHECK_THROWS_AS(copy(src_span_dyn, dst_span_static), fail_fast&);

But I don't think this is the intent and looking at the Catch tests
itself looks like this macro does not expect the reference:

    REQUIRE_THROWS_AS( thisThrows(), std::domain_error );
    CHECK_THROWS_AS( thisDoesntThrow(), std::domain_error );

This commit makes Catch catch exceptions by reference instead of by
value.

include/internal/catch_capture.hpp

@@ -136,7 +136,7 @@
                 static_cast<void>(expr); \
                 __catchResult.captureResult( Catch::ResultWas::DidntThrowException ); \
             } \
-            catch( exceptionType ) { \
+            catch( exceptionType& ) { \
                 __catchResult.captureResult( Catch::ResultWas::Ok ); \
             } \
             catch( ... ) { \